Pediatric acute liver failure (PALF) is a life threatening disorder in which previously healthy children become critically ill secondary to rapid loss of hepatic function. The ongoing, multi-center, NIH-funded PALF Study Group was started in 1999 with the goal of studying the etiology, pathogenesis, diagnosis, prognosis and treatment of acute liver failure in children. Strikingly, approximately 40% of the subjects in the PALF cohor have no final specific etiologic diagnosis (indeterminate) for their hepatic failure. It is likel that unrecognized viral infection may play a critical role in the etiology of liver failure in at least ome of these subjects. In this proposal, state-of-the-art methods for pathogen discovery will be used to evaluate liver and serum specimens from PALF patients for the presence of viruses. This patient cohort is unique, and represents the largest repository of specimens from PALF patients in the world. Specifically, ultra-high throughput Next Generation sequencing will be used in conjunction with a customized bioinformatics analysis pipeline to define the spectrum of viruses, both known and novel, present in the indeterminate cases. Any such viruses detected would be potential candidate etiologic agents of PALF. As a first step towards establishing an association with PALF, case-control analyses will be performed to define prevalence of each of these agents in serum of unexplained PALF cases as compared to PALF cases with non-infectious etiologies as well as to healthy controls.